It's not doable because it'snot designed to work that way. It's one or the other...unless you have an old Nforce board and an 8000 series card. Theres an option that they used for a while called GPU sync or something along those lines that utilized the onboard plus pci-e card....but they pretty much scrapped that after one run.

After reading closer it seems you still can not run both at the same time...Connecting the monitor cable:
a. For i-Mode, please connect the monitor to 1 or more of the video output connector on the desktop board
b. For d-Mode, please connect the monitor to 1 or more of the video output on the graphics card
Note: Please do not connect monitors to the desktop board and graphics card video output connectors simultaneously as this is not a supported Lucid Virtu configuration

After reading closer it seems you still can not run both at the same time...Connecting the monitor cable:
a. For i-Mode, please connect the monitor to 1 or more of the video output connector on the desktop board
b. For d-Mode, please connect the monitor to 1 or more of the video output on the graphics card
Note: Please do not connect monitors to the desktop board and graphics card video output connectors simultaneously as this is not a supported Lucid Virtu configuration
but I think it will still help with temp and load problems you described by using the onboard for less demanding video, anyhow heres some more info on it http://downloadmirror.intel.com/19993/eng/Lucid%20Virtu%20Users%20Guide%20v1%202%20Universal.pdf

So I could plug in both monitors to the motherboard and use PG.

But in games, both would kick in? I.e. GPU and the PG? - Or how would the GPU work when it cannot display anything? Or does that go via motherboard?

The "how to" is rather unclear, what exactly should I do in order to do this?

So I could plug in both monitors to the motherboard and use PG.
But in games, both would kick in? I.e. GPU and the PG? - Or how would the GPU work when it cannot display anything? Or does that go via motherboard?
The "how to" is rather unclear, what exactly should I do in order to do this?

I believe it uses the mobo as a pass through while still using the discreet on games.

U wonder why you are worried about the temps too much.
I run three monitors with crossfire setup where cards sit right on top of each other.
The top card idles at 62c without anu issues.
These cards ca handle the heat so i dont think this is an issue.